N,N′-Bis(aryl)pyridine-2,6-dicarboxamide complexes of ruthenium: Synthesis, structure and redox properties

Article abstract of DOI:10.1016/j.poly.2007.08.042

Reaction of five N,N′-bis(aryl)pyridine-2,6-dicarboxamides (H₂L-R, where H₂ denotes the two acidic protons and R (R = OCH₃, CH₃, H, Cl and NO₂) the para substituent in the aryl fragment) with [Ru(trpy)Cl₃](trpy = 2,2′,2″-terpyridine) in refluxing ethanol in the presence of a base (NEt₃) affords a group of complexes of the type [Ru^II(trpy)(L-R)], each of which contains an amide ligand coordinated to the metal center as a dianionic tridentate N,N,N-donor along with a terpyridine ligand. Structure of the [Ru^II(trpy)(L-Cl)] complex has been determined by X-ray crystallography. All the Ru(II) complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru^II(trpy)(L-R)] complexes shows a Ru(II)-Ru(III) oxidation within 0.16-0.33 V versus SCE. An oxidation of the coordinated amide ligand is also observed within 0.94-1.33 V versus SCE and a reduction of coordinated terpyridine ligand within -1.10 to -1.15 V versus SCE. Constant potential coulometric oxidation of the [Ru^II(trpy)(L-R)] complexes produces the corresponding [Ru^III(trpy)(L-R)]⁺ complexes, which have been isolated as the perchlorate salts. Structure of the [Ru^III(trpy)(L-CH₃)]ClO₄ complex has been determined by X-ray crystallography. All the Ru(III) complexes are one-electron paramagnetic, and show anisotropic ESR spectra at 77 K and intense LMCT transitions in the visible region. A weak ligand-field band has also been shown by all the [Ru^III(trpy)(L-R)]ClO₄ complexes near 1600 nm.

Full text of DOI:10.1016/j.poly.2007.08.042

Available online at www.sciencedirect.com
Polyhedron 27 (2008) 139–150
www.elsevier.com/locate/poly
N,N0-Bis(aryl)pyridine-2,6-dicarboxamide complexes
of ruthenium: Synthesis, structure and redox properties
a
a
b
c
Moutusi Dasgupta , Sumon Nag , Gopal Das , Munirathinam Nethaji ,
a,
*
Samaresh Bhattacharya
a
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b
Department of Chemistry, Indian Institute of Technology, Guwahati 781 039, Assam, India
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
c
Received 16 July 2007; accepted 27 August 2007
Available online 23 October 2007
Abstract
Reaction of five N,N0-bis(aryl)pyridine-2,6-dicarboxamides (H2L-R, where H2 denotes the two acidic protons and R (R = OCH3,
CH3, H, Cl and NO2) the para substituent in the aryl fragment) with [Ru(trpy)Cl3](trpy = 2,20,200-terpyridine) in refluxing ethanol in
the presence of a base (NEt3) affords a group of complexes of the type [RuII(trpy)(L-R)], each of which contains an amide ligand coor-
dinated to the metal center as a dianionic tridentate N,N,N-donor along with a terpyridine ligand. Structure of the [RuII-
(trpy)(L-Cl)] complex has been determined by X-ray crystallography. All the Ru(II) complexes are diamagnetic, and show characteristic
1H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on the [RuII(trpy)(L-R)] complexes shows a
Ru(II)–Ru(III) oxidation within 0.16–0.33 V versus SCE. An oxidation of the coordinated amide ligand is also observed within 0.94–
1.33 V versus SCE and a reduction of coordinated terpyridine ligand within ꢀ1.10 to ꢀ1.15 V versus SCE. Constant potential coulomet-
ric oxidation of the [RuII(trpy)(L-R)] complexes produces the corresponding [RuIII(trpy)(L-R)]+ complexes, which have been isolated as
the perchlorate salts. Structure of the [RuIII(trpy)(L-CH3)]ClO4 complex has been determined by X-ray crystallography. All the Ru(III)
complexes are one-electron paramagnetic, and show anisotropic ESR spectra at 77 K and intense LMCT transitions in the visible region.
A weak ligand-field band has also been shown by all the [RuIII(trpy)(L-R)]ClO4 complexes near 1600 nm.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: N,N0-Bis(aryl)pyridine-2,6-dicarboxamide ligands; Ruthenium complexes; Redox properties
1. Introduction
to mention here that chemistry of the amide ligands is of par-
ticular interest with reference to their role in biological pro-
cesses [3]. For example, the amide linkage plays a key role in
the formation and maintenance of protein architectures,
which are crucial for their performance in biological systems
[4]. Apart from their relevance in biological phenomena,
coordination chemistry of the amide ligands is also interest-
ing with regard to their variable binding modes. The amides
usually display two modes of binding to metal ions, through
the nitrogen (amidate mode) via dissociation of the N–H
proton and through the oxygen (amide mode). While bind-
ing in the amidate mode is known to stabilize metal ions in
their high oxidation states, that in the amide mode is
reported to favor relatively lower oxidation states of a metal
[5]. Interconversion between the amide and amidate modes
The chemistry of ruthenium has been receiving consider-
able attention [1], primarily because of the fascinating redox,
photophysical and photochemical properties exhibited by
complexes of this metal. As properties are dependent mostly
on the coordination environment around the metal center,
complexation of ruthenium by ligands of selected types is
of significant importance, and the present work has origi-
nated from our continued interest in this area in general
[2], and our recent findings on the chemistry of some ruthe-
nium amide complexes in particular [2j]. It may be relevant
*
Corresponding author. Tel.: +91 33 2414 6223; fax: +91 33 2414 6584.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.08.042

140
M. Dasgupta et al. / Polyhedron 27 (2008) 139–150
of binding has been efficiently utilized to manipulate redox
properties of the metal center [6]. In a recent study on a group
of tris-complexes of N-(aryl)picolinamides (1) [2j], where the
amides are bound to ruthenium through the pyridine-nitro-
gen (Np) and the amide-nitrogen (Na) (as in 2) creating a
Np3Na3 coordination sphere, the trivalent state of ruthenium
has been found to be highly stable as reflected in the
Ru(III)–Ru(II) reduction potential (ꢀ0.38 V versus SCE,
for the R = H complex). From the magnitude of this reduc-
tion potential it became evident that the bivalent state of
ruthenium cannot be stabilized in the same Np3N3a environ-
ment. In order to have stable ruthenium(II) complexes of
such amides, a logical approach seems to be to reduce the
number of hard amide nitrogens and increase that of soft
pyridine nitrogens in the coordination sphere. Thus we
have planned to create a Np4Na2 coordination sphere around
ruthenium, and with this simple strategy a group of N,N0-
bis(aryl)pyridine-2,6-dicarboxamides (3), derived from
2,20-dipicolinic acid and five different para-substituted
anilines, have been selected as the ligands and
[Ru(trpy)Cl3](trpy = 2,20,200-terpyridine) as the ruthenium
starting material. The selected amides (3) are known to bind
to metal ions usually as dianaionic tridentate Na,Np,
Na-donors (4) via loss of the two amide protons [7]. How-
ever, there are few instances where they also bind to metal
ions as monoanaionic Na,Np,O-donors as well as neutral
O,Np,O-donors [8]. It may be mentioned here that though
chemistry of amide complexes of many transition metals
has been extensively studied [9], that of ruthenium appears
to have received much less attention [2j,o,7a,10]. As the
source of ruthenium the [Ru(trpy)Cl3] complex has been
selected because of its demonstrated ability to accommodate
tridentate ligands via displacement of three chlorides and
thus providing three pyridine nitrogens in the coordination
sphere [11]. Reaction of the selected amides (3) with
[Ru(trpy)Cl3] has indeed been found to afford a family of
complexes having the targeted Np4N2a coordination sphere
around ruthenium. An account of the chemistry of all
these complexes is presented in this report, with special refer-
ence to their formation, characterization and, spectral and
electrochemical properties.
2. Experimental
2.1. Materials
Commercial ruthenium trichloride was purchased from
Arora Matthey, Kolkata, India. The para-substituted
anilines, 2,20,200-terpyridine and 2,6-dipicolinic acid were
obtained from Aldrich, India. All other chemicals and
solvents were reagent grade commercial materials and
were used as received. [Ru(trpy)Cl3] was synthesized by
following a reported procedure [12]. The N,N0-bis(aryl)pyr-
idine-2,6-dicarboxamides (3, H2L-R) were prepared by
condensing 2,6-dipicolinic acid with para-substituted ani-
lines [10a]. Purification of dichloromethane and acetoni-
trile, and preparation of tetrabutylammonium perchlorate
(TBAP) for electrochemical work were performed as
reported in the literature [13].
2.2. Synthesis
2.2.1. [RuII(trpy)(L-R)]
The [RuII(trpy)(L-R)] complexes were obtained by fol-
lowing a general procedure. Specific details are given below
for a particular complex.
2.2.1.1. [RuII(trpy)(L-H)]. To a solution of the H2L-H
ligand (70 mg, 0.22 mmol) in ethanol (40 mL) was added
triethylamine (70 mg, 0.70 mmol) followed by [Ru(tr-
py)Cl3] (100 mg, 0.22 mmol), and the mixture was then
refluxed for 24 h to yield a brown solution. The solvent
was evaporated and the brown residue, thus obtained,
was subjected to purification by thin layer chromatography
on a silica plate. With 1:1 acetonitrile–toluene as the elu-
ant, a brown band separated, which was extracted with
acetonitrile. Evaporation of this acetonitrile extract gave
[RuII(trpy)(L-H)] as a brown crystalline solid. Yield:
103 mg (70%).
2.2.1.2. [RuII(trpy)(L-OCH3)]. Anal. Calc. for C36H28-
N6O4Ru: C, 60.93; H, 3.94; N, 11.84. Found: C, 60.83;
H, 3.72; N, 11.95%. H NMR in CDCl3, d ppm1: 3.57 (s,
1
6H, OCH3); 5.55 (d, 4H, J = 6.2); 6.57 (d, 4H, J = 6.2);
7.32 (t, 2H, J = 7.8); 7.49–7.61 (4H)*; 7.78 (d, 2H,
J = 7.7); 7.81 (d, 2H, J = 7.7); 8.15 (t, 2H, J = 7.4)*; 8.31
(2H)*.
N
N
N
N
Ru
H
O
O
R= OCH3, CH3, H,
Cl, NO2
2.2.1.3. [RuII(trpy)(L-CH3)]. Anal. Calc. for C36H28-
R
R
N6O2Ru: C, 63.81; H, 4.13; N, 12.40. Found: C, 63.77;
1
2
1
H, 4.04; N, 12.48%. H NMR in CDCl3, d ppm1: 2.10 (s,
6H, CH3); 5.35 (d, 4H, J = 6.1); 6.37 (d, 4H, J = 6.2);
7.34 (t, 2H, J = 7.8); 7.51–7.66 (4H)*; 7.77 (d, 2H,
O
O
O
O
N
N
NH
HN
N
N
M
1
Chemical shifts are given in ppm and multiplicity of the signals along
R
R
R
R
with the associated coupling constants (J in Hz) are given in parentheses.
Overlapping signals are marked with an asterisk.
3
4

M. Dasgupta et al. / Polyhedron 27 (2008) 139–150
141
J = 7.7); 7.79 (d, 2H, J = 7.7); 8.11 (t, 2H, J = 7.4); 8.28
(2H)*.
2.3. Physical measurements
Microanalyses (C, H, N) were performed using a Herae-
us Carlo Erba 1108 elemental analyzer. IR spectra were
obtained on a Perkin–Elmer 783 spectrometer with samples
prepared as KBr pellets. Electronic spectra were recorded
on a JASCO V-570 spectrophotometer. Magnetic suscepti-
bilities were measured using a PAR 155 vibrating sample
magnetometer fitted with a Walker Scientific L75FBAL
magnet. NMR spectra were recorded in CDCl3 solution
with a Bruker AV 300 NMR spectrometer. ESR spectra
were recorded with a JEOL JES-FA200 X-band spectrom-
eter fitted with a quartz Dewar for measurements at 77 K
(liquid dinitrogen). All ESR spectra were calibrated with
an aid of DPPH (g = 2.0037). Electrochemical measure-
ments were made using a CH Instruments model 600A
electrochemical analyzer. A platinum disc working elec-
trode, a platinum wire auxiliary electrode and an aqueous
saturated calomel reference electrode (SCE) were used
in the cyclic voltammetry experiments. A platinum-wire-
gauge working electrode was used in the coulometric exper-
iments. All electrochemical experiments were performed
under a dinitrogen atmosphere. All electrochemical data
were collected at 298 K and are uncorrected for junction
potentials.
2.2.1.4. [RuII(trpy)(L-H)]. Anal. Calc. for C34H24-
N6O2Ru: C, 62.86; H, 3.69; N, 12.94. Found: C, 62.45;
1
H, 3.82; N, 13.01%. H NMR in CDCl3, d ppm1: 5.33 (t,
2H, J = 6.1); 5.54 (t, 4H, J = 6.1); 6.45 (d, 4H, J = 6.2);
7.31 (t, 2H, J = 7.8); 7.43–7.62 (4H)*; 7.80 (d, 2H,
J = 7.7); 7.82 (d, 2H, J = 7.7); 8.15 (d, 2H, J = 7.7); 8.32
(2H)*.
2.2.1.5. [RuII(trpy)(L-Cl)]. Anal. Calc. for C34H22N6-
O2Cl2Ru: C, 56.82; H, 3.06; N, 11.69. Found: C, 56.95;
1
H, 3.10; N 11.85%. H NMR in CDCl3, d ppm1: 5.34 (d,
4H, J = 6.1); 6.48 (d, 4H, J = 6.2); 7.29 (t, 2H, J = 7.8);
7.43–7.62 (4H)*; 7.79 (d, 2H, J = 7.7); 7.86 (d, 2H,
J = 7.7); 8.18 (d, 2H, J = 7.4); 8.26 (2H)*.
2.2.1.6. [RuII(trpy)(L-NO2)]. Anal. Calc. for C34H22-
N8O6Ru: C, 55.20; H, 2.97; N, 15.15. Found: C, 55.30;
1
H, 2.85; N, 15.25%. H NMR in CDCl3, d ppm1: 5.36 (d,
4H, J = 6.1); 6.47 (d, 4H, J = 6.2); 7.33 (t, 2H, J = 7.8);
7.43–7.62 (4H)*; 7.75 (d, 2H, J = 7.7); 7.83 (d, 2H,
J = 7.7); 8.13 (d, 2H, J = 7.4), 8.29 (2H)*.
2.2.2. [RuIII(trpy)(L-R)]ClO4
2.4. X-ray crystallography
The [RuIII(trpy)(L-R)]ClO4 complexes were obtained by
following a general procedure. Specific details are given
below for a particular complex.
Single crystals of [RuII(trpy)(L-Cl)] were obtained by
slow evaporation of an acetonitrile solution of the
complex. Single crystals of [RuIII(trpy)(L-Me)]ClO4 were
obtained by slow diffusion of toluene into an acetonitrile
solution of the complex. Data were collected on a Bruker
SMART APEX CCD diffractometer using graphite
2.2.2.1. [RuIII(trpy)(L-H)]ClO4. A solution of [RuII-
(trpy)(L-H)] (22 mg, 0.03 mmol) in 50 mL of 1:9 dichloro-
methane–acetonitrile (0.1 M TBAP) was coulometrically
oxidized at 0.4 V versus SCE. Color of the solution gradu-
ally changed from brown to green during the oxidation.
After the electrolysis was complete (Qobserved = 3.47 C,
Qcalculated = 3.36 C), the resulting acetonitrile solution was
˚
monochromated and Mo Ka radiation (k = 0.71073 A).
X-ray data reduction and, structure solution and
refinement were done using SHELXS-97 and SHELXL-97 pro-
grams [14]. The structures were solved by the direct
methods.
then layered over toluene. Green crystals of the [RuIII
-
(trpy)(L-H)]ClO4 complex were obtained upon slow diffu-
sion of toluene into the acetonitrile solution, which were
collected manually. The yield was quantitative.
2.2.2.1.1. [RuIII(trpy)(L-OCH3)]ClO4. Anal. Calc. for
C36H28N6O8ClRu: C, 53.43; H, 3.46; N, 10.38. Found: C,
53.78; H, 3.34; N, 10.44%.
Crystal data for C51H33N9O6Cl3Ru1.5, M = 1125.34,
0.33 · 0.34 · 0.45 mm3, monoclinic, space group C2/c,
˚
˚
˚
a = 32.244(7) A, b = 15.797(4) A, c = 19.331(4) A, b =
102.458(4)ꢁ, V = 9615(4) A , Z = 8, Dcalc = 1.556 Mg mꢀꢀ31
,
3
˚
˚
F(000) = 4536, k = 0.71073 A, T = 293 K, l = 0.702 mm
,
49734 reflections collected, 10279 unique (Rint = 0.036).
Final goodness-of-fit = 0.95, R1 = 0.0595, wR2 = 0.2266,
R indices based on 8382 reflections with I > 2r(I) (refine-
ment on F2), 638 parameters.
2.2.2.1.2. [RuIII(trpy)(L-CH3)]ClO4. Anal. Calc. for
C36H28N6O6ClRu: C, 55.63; H, 3.60; N, 10.81. Found: C,
55.92; H, 3.32; N, 10.94%.
2.2.2.1.3. [RuIII(trpy)(L-H)]ClO4. Anal. Calc. for
C34H24N6O6ClRu: C, 54.50; H, 3.20; N, 11.22. Found: C,
54.62; H, 3.63; N, 11.10%.
Crystal data for C36H28N6O6ClRu, M = 813.20,
0.24 · 0.28 · 0.42 mm3, monoclinic, space group P21/c,
˚
˚
˚
a = 14.0441(3) A, b = 21.9700(5) A, c = 12.5956(3) A, b =
2.2.2.1.4. [RuIII(trpy)(L-Cl)]ClO4. Anal. Calc. for
C34H22N6O6Cl3Ru: C, 49.90; H, 2.69; N, 10.27. Found:
C, 49.75; H, 2.77; N, 10.45%.
115.2980(10)ꢁ,
V = 3513.65(14) A ,
Z = 4,
Dcalc =
3
˚
1.537 mg mꢀ3, F(000) = 1660, k = 0.71073 A, T = 273 K,
l = 0.584 mmꢀ1, 40809 reflections collected, 8523 unique
(Rint = 0.032). Final goodness-of-fit = 1.07, R1 = 0.0386,
wR2 = 0.1044, R indices based on 8523 reflections with
I > 2r(I) (refinement on F2), 487 parameters.
˚
2.2.2.1.5. [RuIII(trpy)(L-NO2)]ClO4. Anal. Calc. for
C34H22N8O10ClRu: C, 48.65; H, 2.62; N, 13.35. Found:
C, 48.55; H, 2.80; N, 13.45%.

142
M. Dasgupta et al. / Polyhedron 27 (2008) 139–150
3. Results and discussion
3.1. [RuII(trpy)(L-R)] complexes
dine ring of trpy in a g2-fashion. C–H fragments of the ter-
pyridine and the pendent phenyl ring of the amide are
found to be hydrogen bonded to the chlorine atom in the
amide. One C–H fragment of the terpyridine is also hydro-
gen bonded to the p-cloud in the pendent phenyl ring of the
amide. These hydrogen-bonding interactions appear to be
responsible for holding the complex molecules together in
the crystal lattice. It may be relevant to note here that such
interactions are of significant importance in molecular
recognition processes as well as in crystal engineering
[15]. As all the [RuII(trpy)(L-R)] complexes have been syn-
thesized similarly and they show similar properties (vide
infra), the other four [RuII(trpy)(L-R)] (R ¼ Cl) complexes
are assumed to have similar structure as the [RuII(trpy)-
(L-Cl)] complex.
3.1.1. Synthesis and crystal structure
As delineated in the introduction, the primary objective
of the present study has been to synthesize a group of
mixed-ligand complexes of ruthenium containing both
2,20,200-terpyridine and the N,N0-bis(aryl)pyridine-2,6-
dicarboxamide (3). For this purpose five N,N0-bis(aryl)-
pyridine-2,6-dicarboxamides with different substituents
(R = OCH3, CH3, H, Cl and NO2) at the para position
of the two pendant phenyl rings have been used, in order
to study their influence, if any, on the redox properties of
the resulting ruthenium complexes. These amide ligands
(3) are abbreviated in general as H2L-R, where H2 stands
for the two dissociable protons and R for the substituents.
Reactions of the selected amides (3) with [Ru(trpy)Cl3] pro-
ceed smoothly in refluxing ethanol in the presence of trieth-
ylamine to afford the targeted mixed-ligand complexes of
type [RuII(trpy)(L-R)] in decent yields. It may be noted
here that during the synthetic reaction ruthenium has
undergone a one-electron reduction, and triethylamine
might have served as the reducing agent. Preliminary
(microanalytical, spectroscopic, magnetic, etc.) character-
izations on these complexes (vide infra) are found to be
in well accordance with their compositions. In order to find
out coordination mode of the N,N0-bis(aryl)pyridine-2,6-
dicarboxamides in these complexes, structure of a represen-
tative member of this family, viz. [RuII(trpy)(L-Cl)], has
been determined by X-ray crystallography. The structure
is shown in Fig. 1 and selected bond parameters are listed
in Table 1. The structure shows that the N,N0-bis(40-chlor-
ophenyl)pyridine-2,6-dicarboxamide is coordinated to
ruthenium, via dissociation of two acidic protons, as a
dianionic tridentate N,N,N-donor (4). A terpyridine is also
coordinated to the metal center in the usual fashion.
Ruthenium is therefore sitting in a N6 coordination envi-
ronment, which is distorted octahedral in nature, as
reflected in all the bond parameters around ruthenium.
The observed bond distances in the Ru(L-Cl) fragment
are all quite normal [2j,n], and so are those in the Ru(trpy)
fragment [11a]. In the crystal lattice of the [RuII(trpy)-
(L-Cl)] complex, there are three molecules of water per
two complex molecules. In order to find out the link
between these water molecules and the complex molecule,
packing pattern in the lattice has been scrutinized
(Fig. 2), which shows that besides links with the water mol-
ecules, the complex molecules also have direct links
between themselves. A closer inspection into the network
reveals that three types of hydrogen-bonding interactions,
viz. C–H–O, O–H–O, C–H–Cl and C–H–p, are active in
the lattice. The amide oxygen is linked with one trpy
C–H as well as O–H of water. A C–H fragment from the
pyridine ring of the amide is hydrogen bonded to water
molecule. The central C–H fragment from the pyridine ring
of the amide is linked to the p-cloud over a terminal pyri-
3.1.2. Spectral properties
1H NMR spectra of these complexes show many signals,
most of which are overlapping, and hence assignment of
each signal to a specific proton has not been possible. How-
ever, two distinct doublets observed near 5.3 and 6.3 ppm
in the complexes with R ¼ H, each integrating to four pro-
tons, are assignable to the hydrogens in the two pendent
phenyl rings in the coordinated amide ligand. Signals for
the methoxy and methyl groups in the [RuII(trpy)-
(L-OCH3)] and [RuII(trpy)(L-CH3)] complexes are
observed respectively at 3.57 and 2.10 ppm. The infrared
1
and H NMR spectral data of the [RuII(trpy)(L-R)] com-
plexes are therefore consistent with their compositions.
Infrared spectra of each [RuII(trpy)(L-R)] complex
shows many bands of different intensities in the 400–
4000 cmꢀ1 region. Assignment of each individual band to
a specific vibration has not been attempted. However, com-
parison with spectrum of the respective uncoordinated
ligand shows that the N–H stretch, observed near
3140 cmꢀ1 in the uncoordinated ligands, is absent in the
complexes. The amide C@O stretch, observed near
1680 cmꢀ1 in the uncoordinated ligands, is also found to
be shifted to around 1595 cmꢀ1 in the complexes. Compar-
ison with the spectrum of [Ru(trpy)Cl3] shows the presence
of some common bands (near 648, 670, 729, 781, 1240 and
1444 cmꢀ1) as well as some new bands (near 808, 1008,
1083, 1379 and 1595 cmꢀ1) in the spectra of the [RuII(trpy)-
(L-R)] complexes indicating presence coordinated
terpyridine and amide ligands [8a,2n,j]. The [RuII(trpy)-
(L-R)] complexes are found to be diamagnetic, which is
in accordance with the +2 oxidation state of ruthenium
(low-spin d6, S = 0) in these complexes.
The [RuII(trpy)(L-R)] complexes are soluble in acetone,
acetonitrile, dichloromethane, chloroform, etc., producing
deep brown solutions. Electronic spectra of the complexes
have been recorded in dichloromethane solution. All the
complexes show several intense absorptions in the visible
and ultraviolet regions (Table 2). The absorptions in the
ultraviolet region are attributable to transitions within
the ligand orbitals and those in the visible region are prob-
ably due to charge-transfer transitions. To have a better

M. Dasgupta et al. / Polyhedron 27 (2008) 139–150
143
Fig. 1. (a) Structure of the [RuII(trpy)(L-Cl)] complex and (b) view of the Ru(L-Cl) fragment.
insight into the nature of the absorptions in the visible
region, qualitative EHMO calculations have been per-
formed [16] on the structurally characterized [RuII(trpy)-
(L-Cl)] complex. Partial MO diagrams are shown in
Fig. 3 and composition of selected molecular orbitals is
presented in Table S1 (Supplementary material). The high-
est occupied molecular orbital (HOMO) and the next two
filled orbitals (HOMO ꢀ 1 and HOMO ꢀ 2) have major
(ꢁ60%) contribution from the ruthenium dxy, dyz and dzx
orbitals. These three occupied orbitals may therefore be
regarded as the ruthenium t2 orbitals. The lowest unoccu-
pied molecular orbital (LUMO) has 70% contribution
from the coordinated terpyridine and is dispersed over
the entire ligand. Among the next few vacant orbitals
(LUMO + 1, LUMO + 2, etc.), LUMO + 1 is primarily
centered on the amide ligand and concentrated heavily on
pyridine nitrogen, while LUMO + 2 is delocalized over
both the amide ligand and terpyridine. The lowest energy
absorption, displayed by the [RuII(trpy)(L-R)] complexes
near 540 nm, may therefore be assigned to the charge-
transfer transition taking place from the highest filled
ruthenium t2 orbital (HOMO) to the vacant orbital delo-
calized over the terpyridine (LUMO). The other intense
absorptions in the visible region may be assigned to

144
M. Dasgupta et al. / Polyhedron 27 (2008) 139–150
Table 1
coordination sphere of ruthenium has truly stabilized the
II
˚
Selected bond distances (A) and bond angels (ꢁ) for [Ru (trpy)(L-Cl)] and
bivalent state of the metal to a significant extent, which
has been manifested in the positive shift of the Ru(II)–
Ru(III) potential by about 580 mV.
[RuIII (trpy)(L-CH3)]+
[RuII(trpy)(L-Cl)]
˚
Bond distance (A)
The second oxidation, observed within 0.94–1.33 V, is
quasi-reversible in nature, and is tentatively assigned to
Ru(III)–Ru(IV) oxidation. Based on composition of the
LUMO, the irreversible reduction observed around
ꢀ1.1 V is assigned to reduction of the coordinated terpyri-
dine ligand. Potential of Ru(II)–Ru(III) oxidation has been
observed to be sensitive to nature of the substituent R in
the N,N0-bis(aryl)pyridine-2,6-dicarboxamide. The poten-
tial (E1/2) increases with increasing electron-withdrawing
character of the substituent R. The plot of E1/2 versus 2r
Ru1–N1
Ru1–N2
Ru1–N3
C7–O1
2.098(3)
1.983(4)
2.098(3)
1.249(6)
1.254(6)
Ru1–N4
Ru1–N5
Ru1–N6
2.047(4)
1.946(4)
2.053(4)
C13–O2
Bond angles (ꢁ)
N1–Ru1–N2
N2–Ru1–N3
N1–Ru1–N3
77.22(14)
77.81(14)
155.01(15)
N4–Ru1–N5
N5–Ru1–N6
N4–Ru1–N6
78.64(16)
79.43(15)
158.01(16)
[RuIII(trpy)(L-CH3)]+
[r = Hammett constant of
R
[17]; OCH3 = ꢀ0.27,
˚
Bond distance (A)
CH3 = ꢀ0.17, H = 0.00, Cl = 0.23, NO2 = 0.78] is linear
for these complexes (Fig. S1, Supplementary material) with
a slope (q) of 0.08 (q = reaction constant of this oxidation
[18]). This linear correlation of the Ru(II)–Ru(III) oxida-
tion potentials with the electronic nature of the substituents
having a reasonable slope clearly shows that the substituent
on the phenyl ring of the amide, which is six bonds away
from the metal center, can still influence the metal-centered
redox potential in a predictable manner. Similarly potential
(E1/2) of the Ru(III)–Ru(IV) oxidation also shows linear
correlation with the electron-withdrawing nature (2r) of
substituent R with a slope (q) of 0.18 V (Fig. S1, Supple-
mentary materials). The higher slope of the second plot
indicates that the second oxidation probably involves an
orbital having significant amide character. Potentials of
the irreversible reductive responses do not show any sys-
tematic variation with the nature of the substituent.
Ru1–N1
Ru1–N2
Ru1–N3
C6–O1
1.981(2)
2.023(2)
2.042(2)
1.224(3)
1.235(3)
Ru1–N4
Ru1–N5
Ru1–N6
2.071(2)
1.982(2)
2.080(3)
C14–O2
Bond angles (ꢁ)
N1–Ru1–N2
N1–Ru1–N3
N2–Ru1–N3
80.24(8)
78.81(8)
159.05(8)
N4–Ru1–N5
N5–Ru1–N6
N4–Ru1–N6
79.51(9)
79.19(9)
158.67(9)
charge-transfer transitions from the filled ruthenium t2
orbitals to the higher energy vacant orbitals.
3.1.3. Electrochemical properties
Electrochemical properties of the [RuII(trpy)(L-R)] com-
plexes have been studied by cyclic voltammetry in 1:9
dichloromethane–acetonitrile solution (0.1 M TBAP).2
Each complex shows two oxidative responses at potentials
positive of the SCE reference electrode and a reductive
response at negative potentials (Table 2). A representative
voltammogram is shown in Fig. 4. The first oxidation,
observed within 0.16–0.33 V, is reversible in nature, charac-
terized by a peak-to-peak separation (DEp) of ꢁ70 mV,
which remains unchanged upon changing the scan rate,
and the anodic peak-current (ipa) is almost equal to the
cathodic peak-current (ipc) as expected for a reversible elec-
tron-transfer process. In view of composition of the
HOMO, this oxidation is assigned to Ru(II)–Ru(III) oxida-
tion. It is interesting to note here that in the tris-complexes
of N-(aryl)picolinamides (1) [2j], where ruthenium was
nested in a Np3Na3 coordination sphere, the Ru(III)–Ru(II)
couple appeared on the negative side of SCE (e.g.
ꢀ0.38 V, for the R = H complex), whereas in these
[RuII(trpy)(L-R)] complexes, in which the coordination
environment around the metal center has changed to
Np4Na2, the same couple appears on the positive side of
SCE (e.g. 0.20 V, for the R = H complex). Hence replace-
ment of an amide nitrogen by a pyridine nitrogen in the
3.2. [RuIII(trpy)(L-R)]+ complexes
3.2.1. Synthesis and crystal structure
Reversibility of the Ru(II)–Ru(III) oxidation in the
[RuII(trpy)(L-R)] complexes indicates that the one-electron
oxidized species, viz. [RuIII(trpy)(L-R)]+, might be stable
on a time scale much longer than the cyclic voltammetric
time scale. To investigate this, each [RuII(trpy)(L-R)] com-
plex has been coulometrically oxidized at an appropriate
potential3 in acetonitrile solution (0.1 M TBAP). The oxi-
dation has been smooth and quantitative, associated with
a color change from brown to green, and the oxidized
product has been isolated as [RuIII(trpy)(L-R)]ClO4 in
the solid state. Preliminary characterization data on these
complexes are found to be consistent with their composi-
tions. To examine the difference in structural features, if
any, associated with one-electron oxidation of the metal
center, structure of a selected member of this new family,
viz. [RuIII(trpy)(L-CH3)]ClO4, has also been determined
by X-ray crystallography. The structure (Fig. 5) shows that
apart from small differences in the Ru–N lengths (Table 1),
2
A little dichloromethane was necessary to take the complex into
3
solution. Addition of large excess of acetonitrile was necessary to record
the redox responses in proper shape.
Coulometric oxidation has been carried out at a potential 200 mV
higher than the Epa
.

M. Dasgupta et al. / Polyhedron 27 (2008) 139–150
145
Fig. 2. Packing diagram of the [RuII(trpy)(L-Cl)] crystal.
Table 2
Electronic spectral and cyclic voltammetric data of [RuII(trpy)(L-R)]
Compound
Electronic spectral dataa
k
max, nm (e, Mꢀ1 cmꢀ1
)
Cyclic voltammetric datab
[RuII(trpy)(L-OCH3)]
[RuII(trpy)(L-CH3)]
[RuII(trpy)(L-H)]
236 (20400), 277 (11000),c 314 (9500), 397 (5500), 435 (5000),c 541 (3400)
236 (19400), 278 (10600),c 315 (9400), 396 (4500), 432 (4300),c 540 (2700)
235 (19900), 278 (11500),c 315 (9900), 395 (5100), 430 (5300),c 538 (3500)
233 (19400), 278 (9900),c 314 (7400), 394 (3200), 432 (2300),c 540 (2200)
230 (19200), 277 (14500),c 316 (17800), 326 (17200), 431 (7100),c 541 (3500)
0.16d (70),e 0.94d (69),e ꢀ1.10f
0.18d (74),e 0.99d (90),e ꢀ1.15f
0.20d (63),e 1.05d (96),e ꢀ1.11f
0.22d (76),e 1.15d (93),e ꢀ1.15f
0.33d (66),e 1.33d (62),e ꢀ1.15f
[RuII(trpy)(L-Cl)]
[RuII(trpy)(L-NO2)]
a
In dichloromethane solution.
b
Solvent, 1:9 dichloromethane–acetonitrile; supporting electrolyte, TBAP; scan rate 50 mV sꢀ1
Shoulder.
.
c
d
e
f
E1/2 = 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic peak potentials respectively.
DEp = (Epa ꢀ Epc).
Epc value.
the other features are qualitatively very similar to those of
the [RuII(trpy)(L-R)] complex. The Ru–N distances in the
Ru(L-R) fragment are shorter in the Ru(III) complex,
probably due to enhanced electrostatic attraction of the
dianionic amide ligand with the trivalent metal. The Ru–
N lengths in the Ru(trpy) fragment are, however, slightly
longer in the Ru(III) complexes, which is attributable to
the lack of back-donation from the oxidized metal center
that probably wins over the electrostatic effect. The
packing pattern in the lattice (Fig. 6) shows that

146
M. Dasgupta et al. / Polyhedron 27 (2008) 139–150
Fig. 3. Partial MO diagram of the [RuII(trpy)(L-Cl)] complex.
Fig. 4. Cyclic voltammogram of the [RuII(trpy)(L-Cl)] complex in 1:9
dichloromethane–acetonitrile solution (0.1 M TBAP) at a scan rate of
50 mV sꢀ1
.
Fig. 5. Structure of the [RuIII(trpy)(L-CH3)]ClO4 complex.
hydrogen-bonding interactions of three types, viz. C–H–O,
O–H–O and C–H–p, are active. The perchlorate oxygens
are hydrogen bonded to water and C–H fragment of the
trpy ligand. The amide oxygen is also hydrogen bonded
to water and C–H fragment of the trpy ligand. One C–H
fragment of the trpy ligand is hydrogen bonded to the p-
cloud over the tolyl ring of the amide ligand. Due to such
hydrogen-bonded arrangements of the complex molecules,
several nearly rectangular cavities have been created
throughout the lattice. Based on the similarity in their syn-

M. Dasgupta et al. / Polyhedron 27 (2008) 139–150
147
Fig. 7. ESR spectrum of the [RuIII(trpy)(L-Cl)]ClO4 complex in 1:1
dichloromethane–toluene solution at 77 K.
tude). A selected spectrum is shown in Fig. 7 and the
spectral data for all complexes are given in Table 3. The
anisotropy of the spectra reflects the asymmetry of the
electronic environment around ruthenium in these [RuIII
-
(trpy)(L-R)]ClO4 complexes. When an octahedral geome-
try suffers from an axial distortion (D), the t2 level splits
into a and e components (Fig. 7). Spin–orbit coupling
causes minor additional changes in energy of all the three
levels. Thus two closely spaced electronic transitions (tran-
sition energies DE1 and DE2; DE1 < DE2) are, in principle,
probable within these three levels. All these energy param-
eters have been computed (Table 3) using the observed g
values, the g-tensor theory of low-spin d5 complexes [19],
and a reported method [20]. The axial distortion is found
to be around 6000 cmꢀ1.4 The calculated values of DE1
and DE2 are also close to ꢁ6000 cmꢀ1 (ꢁ1670 nm), which
indicate that two ligand-field transitions, which are close
in energy, should take place in the near-infrared region,
and indeed a broad band of weak intensity has been
observed in the spectra of all the complexes near the pre-
dicted energy (vide infra). Closeness of the two expected
transitions appears to have vitiated their resolution. The
ESR spectral data thus show that these [RuIII(trpy)-
(L-R)]ClO4 complexes are significantly distorted from the
ideal octahedral geometry, as was also indicated by the
structure determination.
Fig. 6. Packing diagram of the [RuIII(trpy)(L-CH3)]ClO4 crystal.
thesis and properties (vide infra), the other four [RuIII
-
(trpy)(L-R)]ClO4 (R ¼ CH3) complexes are assumed to
have similar structures as [RuIII(trpy)(L-CH3)]ClO4.
3.2.2. Spectral properties
Apart from the appearance of two additional intense
bands around 1100 and 620 cmꢀ1 due to presence of the
perchlorate anion and small shifts in the position of the
other bands, infrared spectra of each [RuIII(trpy)-
(L-R)]ClO4 complex is qualitatively similar to that of the
respective [RuII(trpy)(L-R)] complex. Magnetic susceptibil-
ity measurements show that the [RuIII(trpy)(L-R)]ClO4
complexes are one-electron paramagnetic (leff = 1.83–
1.90 lB), which corresponds to the +3 oxidation state of
ruthenium (low-spin d5, S = 1/2) in these complexes. ESR
spectra of the [RuIII(trpy)(L-R)]ClO4 complexes have been
recorded in 1:1 dichloromethane–toluene solution at 77 K.
Each complex shows an axial ESR spectrum with two dis-
tinct signals (g^ and gi; in the order of decreasing magni-
Electronic spectra of the green [RuIII(trpy)(L-R)]ClO4
complexes, recorded in acetonitrile solutions, show absorp-
tions in the near-infrared, visible and ultraviolet regions
(Table 4). The absorptions in the ultraviolet region are
again believed to be due to transitions within the ligand
orbitals. To understand the origin of the absorptions in
the visible region, qualitative EHMO calculations have
been performed on the structurally characterized [RuIII
-
(trpy)(L-CH3)]ClO4 complex. Composition of selected
4
The spin–orbit coupling constant (k) is taken to be 1000 cmꢀ1 for
complexed ruthenium(III) [20].

148
M. Dasgupta et al. / Polyhedron 27 (2008) 139–150
Table 3
ESR g-valuesa and derived parametersb of the [RuIII(trpy)(L-R)]ClO4 complexes
Compounds
g^
gi
D, k
DE1, k
DE2, k
[RuIII(trpy)(L-OCH3)]ClO4
[RuIII(trpy)(L-CH3)]ClO4
[RuIII(trpy)(L-H)]ClO4
[RuIII(trpy)(L-Cl)]ClO4
[RuIII(trpy)(L-NO2)]ClO4
a
2.209
2.217
2.222
2.230
2.236
1.921
1.916
1.914
1.909
1.903
6.219
6.050
5.988
5.836
5.659
5.891
5.727
5.667
5.520
5.349
6.805
6.638
6.577
6.428
6.254
In 1:1 dichloromethane–toluene solution at 77 K.
Spin–orbit coupling constant (k) for complexed Ru(III) is ꢁ1000 cmꢀ1
.
b
Table 4
Electronic spectral data of [RuIII(trpy)(L-R)]ClO4
Compound
Electronic spectral dataa
k
max, nm (e, Mꢀ1 cmꢀ1
)
[RuIII(trpy)(L-OCH3)]ClO4
[RuIII(trpy)(L-CH3)]ClO4
[RuIII(trpy)(L-H)]ClO4
[RuIII(trpy)(L-Cl)]ClO4
[RuIII(trpy)(L-NO2)]ClO4
232 (11200), 273 (5900),b 287 (5200),b 313 (5400), 338 (3500),b 424 (1400), 655 (850), 1553 (112)
231 (10100), 273 (4900),b 282 (4700),b 314 (4800), 332 (4000),b 420 (1300), 652 (1100), 1565 (136)
226 (10000), 276 (6300),b 283 (6400),b 311 (5500), 338 (3400),b 421 (1900), 652 (1500), 1569 (130)
231 (10900), 265 (5800),b 283 (5600),b 314 (5800), 337 (4300),b 425 (2000), 651 (1700), 1576 (143)
226 (11600), 271 (7800),b 283 (8700),b 319 (11200), 340 (8500),b 411 (2300), 646 (1100), 1581 (146)
a
In dichloromethane.
Shoulder.
b
molecular orbitals is given in Table S2 (Supplementary
material) and partial MO diagram is shown in Fig. 8. In
this complex, the HOMO (singly occupied), HOMO ꢀ 1
and HOMO ꢀ 2 are found to have large contributions
(nearly 60%) from ruthenium dxy, dyz and dxz orbitals,
while the HOMO ꢀ 3 has predominant terpyridine charac-
ter. The absorption near 650 nm may therefore be assigned
to ligand-to-metal charge-transfer transition from
HOMO ꢀ 3 to HOMO (singly occupied). The weak
absorption in the near-infrared region (near 1600 nm) has
already been assigned to ligand-field transitions (vide
supra).
a
Fig. 8. Partial MO diagram of the [RuII(trpy)(L-CH3)]ClO4 complex.

M. Dasgupta et al. / Polyhedron 27 (2008) 139–150
149
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3.2.3. Electrochemical properties
Cyclic voltammetric properties of the [RuIII(trpy)-
(L-R)]+ complexes are identical to those of their Ru(II)
precursors, only the Ru(II)–Ru(III) couple appears as a
reductive response. The green [RuIII(trpy)(L-R)]+ com-
plexes could be quantitatively converted back, upon
coulometric reduction at 0.0 V, to the respective brown
[RuII(trpy)(L-R)] complexes, identified by their characteris-
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reduction potentials, these green [RuIII(trpy)(L-R)]+ com-
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4. Conclusions
The present study shows that the N,N0-bis(aryl)pyridine-
2,6-dicarboxamides (3) can efficiently bind to ruthenium(II)
in the dianionic N,N,N-fashion (4) and in combination
with 2,20,200-terpyridine can stabilize the metal in its +2
state. This study further demonstrates that binding by the
amides (3) in the N,N,N-fashion (4) can also sufficiently
stabilize the +3 oxidation state of ruthenium. The coordi-
nation environment provided by the N,N0-bis(aryl)pyri-
dine-2,6-dicarboxamides and 2,20,200-terpyridine appears
to be unique in supporting both the +2 and +3 states of
ruthenium.
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Acknowledgements
(b) F. Basuli, M. Ruf, C.G. Pierpont, S. Bhattacharya, Inorg. Chem.
37 (1998) 6113;
Financial assistance received from the University Grants
Commission, New Delhi, [Grant No. F31-124/2005(SR)] is
gratefully acknowledged. The authors thank Dr. Samar K.
Das, School of Chemistry, University of Hyderabad, India,
for his help with recording the ESR spectra. SN thanks the
Council of Scientific and Industrial Research, New Delhi,
for his fellowship [Grant No. 9/96(401)2003-EMR-I].
(c) A.K. Das, A. Rueda, L.R. Falvello, S.M. Peng, S. Bhattacharya,
Inorg. Chem. 38 (1999) 4365;
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Appendix A. Supplementary material
CCDC 651197 and 651198 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.poly.2007.08.042.
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